Toxicity and accumulation of 6-OH-BDE-47 and newly synthesized 6,6′-diOH-BDE-47 in early life-stages of Zebrafish (Danio rerio)

Dihydroxylated polybrominated diphenyl ethers (diOH-PBDEs) appear to be natural products or metabolites of PBDEs in some marine organisms, yet its toxicity is still largely unknown. With a newly lab-synthesized diOH-PBDE, 6,6′-dihydroxy-2,2′,4′4′-tetrabromodiphenyl ether (6,6′-diOH-BDE-47) in hand, the present study has provided the first data set to compare 6-hydroxy-2,2′,4′4′- tetrabromodiphenyl ether (6-OH-BDE-47) and 6,6′-diOH-BDE-47 for their acute toxicity and accumulation, and thyroid hormone levels in treated zebrafish larvae. By real time-PCR technique, transcripts of hypothalamic-pituitary-thyroid axis associated genes were also investigated in developing larvae at 96 h post fertilization (96 hpf). Apparently, 6,6′-diOH-BDE-47 was less toxic than that of 6-OH-BDE-47: 1) the 96-h LC₅₀ (96-h median lethal concentration) of 6-OH-BDE-47 and 6,6′-diOH-BDE-47 were 235 nM and 516 nM, respectively; 2) although severe developmental delays and morphological deformities were observed in zebrafish larvae in high exposure doses, at the exposure concentration of 1–50 nM, the accumulated 6-OH-BDE-47 and 6,6′-diOH-BDE-47 is ranged between 226–2279 nmol/g and 123–539 nmol/g in treated larvae; and 3) for 6-OH-BDE-47, its bioconcentration factor (BCF) were 1.83- to 4.30-fold more than that of 6,6′-diOH-BDE-47, suggesting that the lower internal exposure concentration of 6,6′-diOH-BDE-47 may lead to lower toxicity. The increased thyroid hormone levels were recorded for 1 nM of 6-OH-BDE-47 and 20 nM of 6,6′-diOH-BDE-47, and the exposures both significantly increased thyroid gland-specific transcription of thyroglobulin gene, indicating an adverse effect associated with the HPT axis. Therefore, 6,6′-diOH-BDE-47, with lower toxicity compared to that of 6-OH-BDE-47, still possesses hazards and environmental risk.